Season the fillet. Heat the clarified butter in a frying pan. Brown the meat all over for approx. 5 mins. on a medium heat, remove.
Finely chop the mushrooms and shallots. Heat the butter in the same pan as the meat. Sauté the onions for approx. 5 mins. Add the shallots and cook briefly.
Add the lemon juice, continue to sauté until the liquid has completely evaporated. Finely chop the parsley, stir in, season and allow to cool.
Prepare the pastry
Sprinkle a little flour on the work surface and roll out the dough to a rectangle of approx. 30 x 45 cm. Cut off a strip of approx. 10 cm on the short edge and leave to cool. This strip will be used as decoration.
Place ¾ of the filling in the centre of the dough, to the same length as the fillet, place the fillet on top then distribute the remaining filling around it.
Wrap up the fillet.
Separate the egg. Beat the egg white. Coat the edges of the dough with egg white, gently place on top of the meat, press down firmly and place on an oven tray lined with baking paper, with the seal facing downwards.
Cut out stars from the strips of dough, brush with egg white, use to decorate the dough and press down lightly.
Cover the fillet and leave to chill for approx. 2 hrs. Thin the egg yolk with a little water. Brush the dough with the yolk, prick firmly, insert the meat thermometer in the thickest part.
Approx. 35 mins. in the lower half of an oven preheated to 220 °C until the core temperature has reached approx. 50°C. Remove the meat and leave to rest for approx. 10 mins. before carving.